In past years I saw islands of bottom paint gone from around through hulls after hauling. I never gave it much technical thought and made the assumption that it was the result of excessive zincs. But of course that would be impossible because copper is more noble than zinc. The problem could have been caused maybe by the bonded metal through hull at 0 volts reacting with the -0.4 copper paint. After giving this topic more thought, I agree that adding more zinc or for that matter aluminum alloy designed for anode applications in salt water will cause no harm.

Carbon brushes. My brushes that ride on my SS shafts are made from machined oilite bearings (oil impregnated sintered bronze). They do work because I have measure their performance. But there is a risk of an oil film developing on the shafts that could reduce the effectiveness of the bond between the two metals just as transmission oil does; that is the reason for shaft bonding.

I do remember seeing what I believed was carbon used for this purpose in commercial shaft bonding products. And yes, carbon is in the most noble or cathodic range on the metal corrosion charts whereas SS in in the mid section between the most and least noble range. But my guess is that teh rotating shaft would provide a wiping action that would polish the bond between the carbon and SS. My shafts are 1 1/2" in diameter. It would take many lifetimes to weaken such mass. So there may be a reason for making a trade off between potential corrosion and providing a better shaft bond.